Sky...this is not all that uncommon a problem. Many people have one arm bigger than the other, or one leg, or, as in your case, a lat. This situation can occur for several reasons...the lack of coordination on one side of your body may impair your ability to perform single arm movements equally as well on each side...if one side of your body is stronger than the other and you perform too many bi-lateral movements, the stronger arm will "take more of the weight" than the weak side, leading to more development on the stronger side...also, some people have slightly impaired nerve transmission to one side of their bodies or to individual muscles.This will result in less muscle fiber recruitment, and therefore, less hypertrophy in that muscle.
My suggestions...(1) focus extra hard mentally in feeling every exercise in the smaller lat. Extra focus can lead to better fiber recruitment (2) use some uni-lateral movements in you training. Single arm rows, single arm pulldowns, and dumbell rows(and perhaps perform an extra set or two with the weaker side to give it extra work). Muscles trained this way will receive more stimulation (3) at the end of your lat workout, stretch out the smaller lat vigorously...this will help it recover, and will stretch the surrounding fascia.
Try these things and you just may equal your lats out in a few months.